In the past, oil country tubular goods had been inserted into the wells and used as is, but in recent years technology has been developed for use after expansion 10 to 20% in the wells. This has greatly contributed to the reduction of oil well and gas well development costs. However, if tensile plastic strain is introduced in the circumferential direction due to the expansion, the yield strength with respect to the compressive stress in the circumferential direction due to outside pressure (hereinafter referred to as the “compression yield strength”) will drop and the pressure at which the steel pipe collapses due to outside pressure (hereinafter referred to as the “collapse pressure”) will drop. This, as is well known as the Bauschinger effect, is the phenomenon where, after plastic deformation, if applying stress in the opposite direction to the direction in which plastic strain was applied, yield occurs by a stress lower than before plastic deformation.
The Bauschinger effect occurs due to plastic stress, so a method for restoring the reduced compression yield strength by heat treatment has been disclosed in Japanese Unexamined Patent Publication (Kokai) No. 9-3545 and Japanese Unexamined Patent Publication (Kokai) No. 9-49025 and reported in numerous research papers. However, if expanding pipe in a well, later high temperature heat treatment is not possible in the well, so steel pipe with little drop in collapse strength after expansion has been sought.